In cryogenic liquefying/refrigerating apparatus of prior art, the compressor is positioned in room temperature environment, and gas-to-be-liquefied must be cooled to its liquefying temperature, i.e. boiling temperature (for example, about −269° C. in the case of helium) in the cooling section, so temperature difference is very large and refrigerating efficiency of the apparatus is remarkably low as compared with usual refrigerating machines. Therefore, a cooling medium (supplementary cooling medium) is introduced from outside the system in order to increase refrigerating efficiency. In the case of helium liquefying/refrigerating systems, liquid nitrogen is widely used as the supplementary cooling medium.
As a cycle for liquefying helium is known a closed cycle using helium as a refrigerant and a system capable of performing the cycle is disclosed in Japanese Laid-Open Patent application No. 60-44775.
FIG. 5 is a schematic diagram of the system disclosed in the above-mentioned JP 60-44775. In the drawing, reference numeral 01 is a heat-insulated cold box maintained under vacuum, reference numerals 02 to 06 are a first to fifth stage heat exchangers arranged in the cold box 01, 07 and 08 are respectively a first and a second expansion turbine, 09 is a Joule-Thomson (J/T) expansion valve, 010 is a gas-liquid separator for separating liquid helium from a mixture of liquid/gas helium. Reference numeral 012 is a compressor, 013 is a high pressure line, 014 is a low pressure line, 015 is a turbine line, and 016 is a precooling line in which liquid nitrogen flows for cooling the compressed helium gas.
In the helium liquefying/refrigerating apparatus of the prior art, high pressure high temperature helium gas discharged from the compressor 012 flows into the high pressure line 013 of the first stage heat exchanger where the helium gas is cooled by heat exchange with the liquid nitrogen flowing in the precooling line 016 and with helium gas flowing in the low pressure line 014, then flows through the high pressure line 013 of the second stage heat exchanger 03 to be further cooled. A portion of the high pressure helium gas which flowed out of the second heat exchanger 03 flows into the first expansion turbine 07, and the remaining portion flows through the high pressure line 013 of the third stage heat exchanger 04 to be further cooled, further flows through the fourth stage heat exchanger 05 and fifth stage heat exchanger 06 to be further cooled and flows into the J/T expansion valve 09.
The helium gas which entered the first expansion turbine 07 expands adiabatically therein to be rendered medium in pressure and low in temperature, then enters the second expansion turbine 08 after cooling helium gas flowing in the low pressure line 014 of the third stage heat exchanger 04, further expands in the second expansion turbine 08 to be rendered low in pressure and temperature, then flows into the low pressure line 014 of the fourth stage heat exchanger 05, thereby maintaining low helium gas temperature in the low pressure line 014. The high pressure low temperature helium gas reached the J/T expansion valve 09 experiences Joule-Thomson expansion there and partly liquefied, liquid helium 011 is stored in the gas-liquid separator 010, and remaining low pressure low temperature helium gas returns to the compressor 012 through the low pressure line 014 passing through the heat exchangers 06˜02.
Japanese Laid-Open Patent application publication No. 10-238889, hereinafter patent literature 2, discloses a helium liquefying/refrigerating system in which an independent variable speed gas turbine electric generating system capable of efficient capacity control of a group of electric motor driven multi-stage compressors is added to a helium liquefying/refrigerating system mentioned above, thereby making it possible to utilize the cold source of the system and to recover waste heat of the system. The system comprises a gas turbine electric generating section including a frequency converter, a fuel supplying section, and a chemical refrigerating system, the chemical refrigerating system being composed to supply cold energy to the heat exchangers of the system utilizing waste gas of the gas turbine electric generating section as a heat source and the fuel supplying section comprising a heating device for gasifying a portion of liquefied natural gas supplied from a liquefied natural gas tank and a vaporizing section for supplying cold energy corresponding to latent heat of vaporization of the liquefied natural gas.
With the construction, improvement in thermal efficiency of the system is aimed at by generating electric power of optimal frequency and of homogeneous wave shape accommodating the combination of the group of multi-stage compressors so that each of induction motors for driving the compressors is driven at rotation speed to meet the demand from the load side thereby achieving optimal efficiency of the compressors, and by providing the gas turbine electric generating section using natural gas, for example, liquefied natural gas, the fuel supplying section, and the chemical refrigerating machine thereby combining the vaporizing section in which cold energy corresponding to latent heat of vaporization of the liquefied natural gas is generated and the chemical refrigerating machine in which cold energy is generated by utilizing waste heat of the gas turbine electric generating section.